1. Field of the Invention
The present invention relates to a ring segment of annular form which is disposed around the outer periphery of the moving blades in a gas turbine.
2. Description of the Related Art
FIG. 8 shows a turbine 1 in section, which is a gas turbine. In this turbine 1, gas at high temperature which has been generated by a combustors (not shown in the figures) is supplied in the direction shown by the arrow 2 and blows against moving blades 3 and 4 so as to rotate the moving blades 3 and 4, and the heat energy in the high temperature gas is converted in this manner to mechanical rotational energy of the moving blades 3 and 4, so as to generate drive power.
The moving blades 3 and 4 are fixed to a mounting platform 5 which is fitted around a main shaft (not shown in the figure). A plurality of these moving blades 3 and 4 are provided around the main shaft, spaced apart along its peripheral direction. They receive the impact of the high temperature gas which flows from the upstream side (the left side in FIG. 8) to the downstream side, and rotate along with the platform 5. Stationary blades 6 and 7 are provided at the upstream sides of the moving blades 3 and 4 respectively. A plurality of these stationary blades 6 and 7 are provided just like the moving blades 3 and 4, they are arranged around the main shaft, spaced apart around its peripheral direction. Furthermore, ring segments 8 are provided around the outer peripheries of the moving blades 3 and 4, with almost constant gaps f being present between these ring segments 8 and the moving blades 3 and 4. The ring segments 8 compose a plurality of individual units 8a (refer to FIG. 10) which are made out of cobalt alloy.
FIG. 9 is a cross sectional view showing one of the moving blades 3 of the turbine 1 and the vicinity of its peripheral portion including one of the ring segments 8. As shown in FIG. 9, a flow conduit 11 is formed through the blade ring 9 so as to open towards the ring segment 8. Furthermore, isolating rings 10 are fitted in the blade ring 9. Air, which is injected either from an air supply source provided externally to the turbine 1 or from a compressor (not shown in the figure), flows into this flow conduit 11 in the direction shown by the arrow 12. An impingement plate 13 and the ring segment 8 are fixed to the isolating rings 10. The impingement plate 13 is provided between the blade ring 9 and the ring segment 8, and it is provided around its circumferential surface with a plurality of cooling apertures 14 for conducting air which is ejected from the flow conduit 11. The ring segment 8 has two flanges 16 upon its outer peripheral surface 15, one at its upstream side and one at its downstream side, and the ring segment 8 is fixed to the isolating rings 10 via these flanges 16. A plurality of cooling conduits 17 are provided to the ring segment 8, each being pierced through the inner portion of the ring segment 8 from the upstream side of its outer peripheral surface 15 to its end surface in the downstream direction.
FIG. 10 is a perspective view of the individual units 8a which make up the ring segment 8. As shown in FIG. 10, each of the flanges 16 extends around the peripheral direction. A roughly rectangular concave portion 19 is provided upon the outer peripheral surface 15 between these flanges 16. A plurality of opening aperture portions 17a of the cooling conduits 17 are provided at the upstream side of this concave portion 19, arranged along the peripheral direction. Furthermore, grooves 21 are formed at each of the side edges 20 of this individual unit 8a, so as to face the adjacent individual units 8a. The impingement plate 13 is arranged around the outer circumferential side of the individual units 8a. A cavity 22 is defined by this impingement plate 13 and the concave portion 19 of the individual unit 8a. 
FIG. 11 is a cross sectional view of prior art ring segment 8 as seen from the axial direction of the main shaft. As shown in FIG. 11, adjacent individual units 8a are linked in the peripheral direction by a seal plate 23 being inserted into both the two grooves 21 which are formed in their mutually confronting side edges 20, so that collectively the individual units 8a constitute an annular ring segment 8. These seal plates 23, along with joining each adjacent pair of individual units 8a together, also serve to prevent the leakage of air and high temperature gas through the gaps between the adjacent pairs of individual units 8a. The thickness of the thin plate portion of each of the individual units 8a is approximately 6 mm. By this thickness of the thin plate portion of each of the individual units 8a is meant the distance (shown in the figure by the symbol d) from the bottom surface of its concave portion 19 to the surface 24 on the other side of the individual unit 8a, which surfaces 24, in cooperation, define the inner peripheral surface 24 of the ring segment 8.
When the turbine is operating, each of the individual units 8a expands both in the peripheral direction and in the axial direction, due to exposure to the influence of the flow of high temperature gas. In consideration of the amount of dimensional variation of the individual units 8a due to thermal expansion in the peripheral direction, a gap e of a few millimeters is provided between each of the individual units 8a and the adjacent one.
Next, the flow of high temperature air and gas during operation of the gas turbine will be explained.
The high temperature gas flows along the direction of the axis of the main shaft as shown by the symbol 2 in FIGS. 8 through 10 and drives each of the moving blades 3 and 4. Furthermore, air is blown and passes through the blade ring 9 for cooling each of the individual units 8a of the ring segment 8. This air flows in the direction shown by the arrow A in FIGS. 9 and 10, and flows into each of the cavities 22 through those of the cooling holes 14 in the impingement plate 13. This air which has flowed into the cavity 22, after having collided with the concave portion 19 and having thereby cooled the ring segment 8, flows in the direction shown by the arrow B, and enters through the opening aperture portions 17a into the cooling conduits 17. And this air which has entered into the cooling conduits 17 flows to the downstream side through the cooling conduits 17 while further cooling the inside of the ring segment 8, finally being ejected from the downstream ends of the cooling conduits 17 into a high temperature gas.
Moreover, this air is blown out at a higher pressure than that of the high temperature gas, in order for none of this high temperature gas to flow into the downstream ends of the cooling conduits 17. When in this manner the air is blown out at a higher pressure than that of the high temperature gas, the seal plate 23 is pressed against the lower surface 25 of the grooves 21 by the pressure difference between the air and the high pressure gas, and thereby the sealing efficiency of the ring segment 8 is enhanced. Due to this, loss of driving power of the gas turbine due to leakage of air and high temperature gas is prevented. However, when the air is thus blown out at a suitable pressure, the high temperature gas intrudes between the seal plates 23 and the grooves 21 from the gaps e between the adjacent pairs of individual units 8a, and the corner edge portions 26 which are delimited between the inner peripheral surfaces 24 and the side edges 20 are each heated up from three sides: the inner peripheral surface 24, the side edge 20, and lower surface 25 of the groove 21. These heated up corner edge portions 26 reach high temperatures locally, and undesirably suffer deterioration due to the occurrence of high temperature oxidation. Furthermore, even if the air is blown out at a suitable pressure, since the corner edge portions 26 are heated up by the high temperature gas which is flowing along the inner peripheral surface 24 and also by the high temperature gas which insinuates into the gaps e between adjacent ones of the individual units 8a, accordingly they can easily suffer high temperature oxidation, and there is a danger that they may be damaged. Yet further, in some cases, the seal plates 23 suffer temperature deformation as well, due to their lower surfaces being directly exposed to the high temperature gas.
If the corner edge portion 26 or the seal plate 23 suffers injury or damage, a large quantity of air will flow out into the high temperature gas side from the corresponding gap e between the adjacent individual units 8a. Furthermore, if the air is no longer being sucked out at a suitable pressure, the high temperature gas may flow out to the outer peripheral side of the ring segment 8 via the gap e. If the high temperature gas or the air leaks in this manner, the gas turbine will suffer an undesirable loss of driving power, and its operational performance will be deteriorated.
Furthermore, with the above described ring segment 8, although the thermal expansion of the individual units 8a in the peripheral direction is approximately absorbed by the gaps e, their thermal expansion in the axial direction is not absorbed, due to each of the flanges 16 being fitted to the blade ring 9 with no gap therebetween, and the peripheral surface of the ring segment 8 between the flanges 16 may suffer warping and may collide with the moving blades 3 and 4.
The present invention has been made in consideration of the above described circumstances, and an object of the present invention is to provide a ring segment for a gas turbine, which is sufficiently well cooled by the flow of air, and which moreover can prevent loss of driving power of the gas turbine.